1. Field of the Invention
The present invention relates to a driver/transmitter for a proximity probe. More particularly, the present invention relates to such a driver which is field programmable.
2. Description of the Prior Art
Rotating machinery such as motors, generators, and turbines find widespread application in areas such as manufacturing, power generation, materials processing, as well as many others. Over time, such machinery is subject to wear and potential failure. Given the high operating revolutions per minute and high power dissipation of many industrial applications of such machinery, failure during operation may have severe consequences in terms of damage to the failed equipment itself as well as neighboring equipment and areas of the installation. In addition, preventive machinery shut downs for maintenance and repair can be very costly in terms of facility downtime and direct expense in labor and replacement parts. Accordingly, it has become important in this area to provide monitoring equipment associated with such rotating machinery to provide indications of its condition.
Some condition monitoring systems utilize eddy current proximity probes. Eddy current probes are well known for their ability to detect the position or condition of varying types of conductive materials. These probes are useful in a variety of related applications including position measurement, such as axial and radial runout or displacement of a rotating assembly. As illustrated in FIG. 1, a proximity probe system has been used to detect the lateral position 15 of a rotating shaft 10 in relation to its journal bearing 11 by mounting one or more probes 12 within the bearing in close proximity to the shaft. The probe is coupled by cable 13 to electronics unit 14.
Eddy current probes comprise an inductor, or coil, situated at the probe tip which is driven with a radio frequency (RF) signal which in turn creates a varying electromagnetic field in any adjacent conductive target material. This electromagnetic field produces eddy currents in the material that induce a counter-electromotive force (emf) in the eddy probe inductor, thereby altering the effective impedance of the inductor. The impedance of the probe therefore provides an indication of the distance between the target and the probe.
Conventionally, the RF oscillator which drives the eddy current probe circuit is an analog transistor-based oscillator such as the Colpitts oscillator. A Colpitts oscillator utilizes a transistor in conjunction with an LC tank circuit wherein the eddy current probe coil functions as the inductive element of the tank circuit A portion of the current flowing in the tank circuit is fed back to the base of the oscillator transistor. The distance between the conductive target area and the probe is often referred to as the “gap,” and varying the gap varies the impedance of the detector coil and thereby varies the output frequency and voltage of the oscillator.
In order to function properly, the electronics unit 14 must be calibrated with the characteristics of the probe 12 and cable 13. Since there are several proximity probes manufacturers, and since the characteristics of the various probes and cables vary from manufacture to manufacturer, the utilization of one manufacturer's probe and cable with another manufacture's electronics package is not feasible because inaccurate data will be obtained. Further, even a given manufacturer may provide several different packages where each such package comprises a probe, cable and electronics unit. The proximity probe and cable from one such package cannot be utilized with the electronics unit from another package without obtaining inaccurate data. Nonetheless, probes, cables and electronics unit are routinely misapplied in the field.
Moreover, even a probe, cable and electronics package that has been factory calibrated to work with one type of target material may not produce accurate results in the field for the same type target material. This is due to differences in material characteristics between the target material in the factory and the target material in the field.
U.S. Pat. No. 5,854,553 discloses a so-called phase lock system, which should work in the application of FIG. 1, but only if the system has a narrow hold-in frequency range. Narrow hold-in frequency range means that system could be adjusted at the factory to work with particular cable length, material and probe type. But a system with narrow hold-in frequency range would not work with different cables, probes and materials. A system as in U.S. Pat. No. 5,854,553 with wide hold-in frequency range could not be applied because theoretically and practically it has the self-oscillation of the output signal and has no chance to be used for the application.
U.S. Pat. No. 6,664,782 (and others which this author has) is based on the premise of storing all possible material characteristics in a memory and using probe impedance measurements and a mathematical algorithm to automatically adjust the system to the correct gap measurements. This system could work with materials whose characteristics are downloaded in advance in a memory. Also, the system may work only with the cables and probe types provided by the company which manufactures the whole system and could not be field-calibrated with different probe or cable type made by other manufacturers. Additionally, as mentioned above, the characteristics of real materials in the field might be different from factory targets and this difference creates additional errors in a field for the above system.
Those skilled in the art would find it advantageous to have an electronics package that is capable of being calibrated to work with any proximity probe and cable and that is capable of being programmed in the field. These novel and useful results have been achieved by the driver/transmitter of the present invention.